Papers by Shirish Nagaraj
Purpose For consideration by 802.20 as it evaluates proposals for FDD MBWA. Notice This document ... more Purpose For consideration by 802.20 as it evaluates proposals for FDD MBWA. Notice This document has been prepared to assist the IEEE 802.20 Working Group. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein. Release The contributor grants a free, irrevocable license to the IEEE to incorporate material contained in this contribution, and any modifications thereof, in the creation of an IEEE Standards publication; to copyright in the IEEE’s name any IEEE Standards publication even though it may include portions of this contribution; and at the IEEE’s sole discretion to permit others to reproduce in whole or in part the resulting IEEE Standards publication. The contributor also acknowledges and accepts that this contribution may be made public ...
2016 24th European Signal Processing Conference (EUSIPCO), 2016
We consider the problem of 3D multiple-inputmultiple-output (MIMO) orthogonal frequency-division ... more We consider the problem of 3D multiple-inputmultiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) channel estimation, a key for the future development of 3D beamforming techniques. Our main contribution is a novel algorithm, namely the adaptive-LASSO, that can jointly exploit the sparsity structure of the MIMO-OFDM channel in the spatial and delay domains. The algorithm is designed to handle large antenna arrays by means of a hybrid analog-digital architecture. In this regard, we describe an effective beamswitching strategy to sample the channel using a few analog beamformers. We investigate the impact of the signal bandwidth, antenna structures, line-of-sight (LOS) and non-line-of-sight (NLOS) conditions via ray-tracing based simulations. Also, we show that the A-LASSO can provide significant improvements with respect to the legacy methods, e.g. least-square technique.
Impact of AWGN Channel on LTE RACH Throughput
2015 IEEE 82nd Vehicular Technology Conference (VTC2015-Fall), 2015
As the 3GPP Long Term Evolution (LTE) systems mature and more LTE mobiles become ubiquitous the R... more As the 3GPP Long Term Evolution (LTE) systems mature and more LTE mobiles become ubiquitous the Random Access Channel (RACH) will experience higher load. Understanding RACH capacity is of high importance for the 4G system planners. Capacity of the LTE RACH was analyzed in the past under the assumption of the ideal receiver with zero missed detection probability. In this work we assume realistic PRACH receiver under the assumption of AWGN channel which is specified as one of the test cases in the standard. We derive the expression for the preamble detection probability when r UEs are transmitting. Using this quantity we compute normalized RACH throughput for a single cell. We experiment with different parameters to illustrate their impact on RACH capacity. We show that the Gaussian channel can degrade ideal receiver capacity significantly (13%-66%) depending on the choice of system parameters.
Method of controlling load sharing between in-cell and soft handoff users
Uplink grouping and aperture apparatus
Method and System for Prediction and Root Cause Recommendations of Service Access Quality of Experience Issues in Communication Networks
Antenna verification for closed loop transmit diversity in UMTS
IEEE 60th Vehicular Technology Conference, 2004. VTC2004-Fall. 2004
We discuss the problem of estimating, at the mobile receiver, the weights applied by the base sta... more We discuss the problem of estimating, at the mobile receiver, the weights applied by the base station transmitter when closed loop transmit diversity is used. This problem is referred to as antenna verification in the UMTS literature and performance of any closed loop scheme depends greatly on the accuracy of such an estimation. Maximum a posteriori and minimum mean square
Method of determining random access channel preamble detection performance in a communication system
Method and apparatus for providing random access detection in a communication system
Method of scheduling mobile user transmissions and methods of decoding mobile user transmissions
Methods for reducing power consumption at transmitters and receivers
재-전송 표시자 및 문턱치에 기초하여 전송된 데이터를 전달하는 데이터 채널에 대응하는 제어 채널을 처리할지의 여부는 수신기에서 결정된다. 재전송 표시자는 전송된 데이터가 전송... more 재-전송 표시자 및 문턱치에 기초하여 전송된 데이터를 전달하는 데이터 채널에 대응하는 제어 채널을 처리할지의 여부는 수신기에서 결정된다. 재전송 표시자는 전송된 데이터가 전송된 다수의 횟수들을 나타낸다. 그 다음, 제어 채널 상에 수신된 제어 정보는 결정 단계에 기초하여 선택적으로 처리된다. 재-전송 표시자, 데이터 채널, 제어 채널
2015 IEEE International Conference on Communications (ICC), 2015
We address the problem of uplink cooperative reception with constraints on both backhaul bandwidt... more We address the problem of uplink cooperative reception with constraints on both backhaul bandwidth and the receiver aperture, or number of antenna signals that can be processed. The problem is cast as a network utility (weighted sum rate) maximization subject to computational complexity and architectural bandwidth sharing constraints. We show that a relaxed version of the problem is convex, and can be solved via a dual-decomposition. The proposed solution is distributed in that each cell broadcasts a set of demand prices based on the data sharing requests they receive. Given the demand prices, the algorithm determines an antenna/cell ordering and antennaselection for each scheduled user in a cell. This algorithm, referred to as LiquidMAAS, iterates between the preceding two steps. Simulations of realistic network scenarios show that the algorithm exhibits fast convergence even for systems with large number of cells.
A minimax approach to open-loop downlink beamforming
ISCAS 2001. The 2001 IEEE International Symposium on Circuits and Systems (Cat. No.01CH37196), 2001
A new method for downlink beamforming is analyzed in this paper. In this open-loop solution, a mi... more A new method for downlink beamforming is analyzed in this paper. In this open-loop solution, a minimax algorithm based on optimal set membership filtering is utilized to synthesize a wide and power efficient beam. It is shown that the robustness of this method against angle spread and error of DOA estimation is greater than that of the classical methods
Performance of Co-Operative Uplink Reception with Non-Ideal Backhaul
2015 IEEE 81st Vehicular Technology Conference (VTC Spring), 2015
In this paper, we consider the problem of uplink coordinated multi-point (UL CoMP) in a wireless ... more In this paper, we consider the problem of uplink coordinated multi-point (UL CoMP) in a wireless network with non-ideal backhaul links. UL CoMP with joint receivers has the potential to improve both cell-edge and system capacity depending on the deployment environment. Realizing the performance gains from CoMP requires ideally large bandwidth and low-latency backhaul links to do data exchange. Even if backhaul bandwidth is well-provisioned, latencies in the backhaul data exchange protocols could lead to severe performance degradation due to the requirement of strict Ack/Nack timing imposed by the LTE standard [1]. In order to deal with this issue, we propose strategies for adaptive helper cell management in order to entail minimal performance loss across different loading conditions. Using system simulations and detailed performance analysis, we show the advantage of using the proposed approaches with a finite buffer traffic model for a heterogeneous network deployment scenario including a Macro-cell layer and a clustered small-cell layer.
Multi-cell distributed interference cancellation for Co-operative Pico-cell clusters
2012 IEEE Global Communications Conference (GLOBECOM), 2012
ABSTRACT Alternative wireless topologies, e.g., an underlay network of Pico-cells, are increasing... more ABSTRACT Alternative wireless topologies, e.g., an underlay network of Pico-cells, are increasingly seen as being necessary to enhance capacity and coverage in next generation wireless systems. Such networks of closely-spaced base-sites have to contend with high interference issues, wherein interference mitigation via power control or interference co-ordination may not be sufficient. Capacity can be significantly enhanced by Co-ordinated Multi-Point (CoMP) techniques, which allow co-operative signal processing to jointly decode user's signals, especially given fast communication links between the nodes. In this paper, we present a cooperative system design for the uplink of LTE for providing high capacity solutions in dense deployments. We propose non-linear receiver algorithms for inter-cell interference suppression and cancellation based on a decentralized multistage cancellation architecture using user-specific clustered Pico-cell antennas. An interference cancellation method acting on post-antenna combined signal is proposed to reduce the computational complexity of the receiver. The decoding latency inherent in multistage receivers is another key bottleneck, which is addressed in this paper by a method to predict the ACK/NACK status at the output of the multistage receiver before actually doing the cancellation. Simulation results for a dense stadium deployment of Pico-cells are presented that show significant capacity gains with the proposed approaches.
Interference Cancellation Based DFT-Precoded CDMA for Hybrid OFCDMA Multiple Access
IEEE GLOBECOM 2007-2007 IEEE Global Telecommunications Conference, 2007
A novel hybrid OFDMA/CDMA multiple access system for uplink communications in evolving 4G wireles... more A novel hybrid OFDMA/CDMA multiple access system for uplink communications in evolving 4G wireless systems is developed. Specifically, a DFT-precoded CDMA technique is proposed for the CDMA part of the multiple access. We elaborate on different configurations of precoded CDMA operation, and develop advanced receiver techniques, using frequency-domain interference cancellation. Detailed simulation results showing the performance of this multiple access approach is presented. Finally, we describe how the proposed approach has been incorporated in the currently developing Ultra Mobile Broadband (UMB) system, which is an evolution from the cdma2000 evolution standards family.
2014 IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP), 2014
In this paper the up-link channel estimation problem in a multi-cell network with a Single-Input-... more In this paper the up-link channel estimation problem in a multi-cell network with a Single-Input-Multiple-Output (SIMO) fading correlated channel and finite-length, nonorthogonal and contaminated pilots is considered. Our contribution is a novel channel estimation technique, referred to as the Sparse Channel Estimator (SCE), which exploits the Karhunen-Loéve Transform (KLT) sparse-representation of the channel. We show that, by simply utilizing an interferencecontaminated channel sample-covariance matrix, the proposed approach outperforms Least Squares (LS)-based and Minimum Mean Squared Error (MMSE) techniques relying on the same type of assumptions.
Performance of pico-cell clusters with cooperative receivers
2012 IEEE Global Communications Conference (GLOBECOM), 2012
ABSTRACT Pico-cell clusters have been proposed for providing high capacities to dense sets of use... more ABSTRACT Pico-cell clusters have been proposed for providing high capacities to dense sets of users in next generation networks. We consider the uplink channel and analyze the sum-rate performance of two interfering pico-cell clusters with cooperative multi-cell joint decoding (MJD). The performance of a linear receiver is compared with that of non-linear receivers which allow message passing between the clusters in order to cancel the associated interference. The receiver, in all the above cases, jointly processes all signals across a single cluster. However we consider different message passing schemes, and maximize the sum rate over system parameters, including the load in each cell. Our results are valid in the large-system limit as the number of cells and antennas per cluster each tend to infinity with fixed ratio. Numerical results show that the nonlinear receivers provide significantly larger capacities than linear receivers, especially at high SNRs.
Coordinated beamforming in clustered HetNets: System design and performance evaluation
2014 IEEE Wireless Communications and Networking Conference Workshops (WCNCW), 2014
In this paper, we consider interference issues arising in HetNets where the small-cells are deplo... more In this paper, we consider interference issues arising in HetNets where the small-cells are deployed in closely-spaced clusters. Such dense deployments pose an important challenge for scaling throughput proportional to the number of cells, since inter-cell interference becomes a key bottleneck for achieving high capacity. We examine key design issues around the use of coordinated beamforming (CBF) strategies for mitigating inter-small-cell interference in such clustered layouts. In particular, we show that system planning of the sounding reference signal (SRS), along with multi-user channel estimation approaches, are critical to realize the benefits of CBF. We also develop key notions of beneficiary sets and co-operative clusters for enabling such interference mitigation in cellular networks. Through extensive and detailed system simulations, we showcase the benefits of the proposed system designs for two use cases: (1) a dense urban Micro-cellular deployment, and (2) a clustered small-cell Stadium deployment. System parameter optimization of SRS power control and co-operative cluster size are also developed. Significant capacity gains are observed for both these deployments using the proposed system design in clustered small-cell topologies.
Lab Performance Analysis of a 4G LTE Prototype
2009 IEEE Wireless Communications and Networking Conference, 2009
Shirish Nagaraj, Sanjeev Garg, Frank Liang, Weidong Yang, Nitin Mangalvedhe, John Haug, KV Pradap... more Shirish Nagaraj, Sanjeev Garg, Frank Liang, Weidong Yang, Nitin Mangalvedhe, John Haug, KV Pradap Home and Networks Mobility Motorola, Inc. Arlington Heights, IL [email protected] ... Abstract The uplink performance of a 4G LTE system, ...
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Papers by Shirish Nagaraj